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Technical Description of the CAMx Photochemical Model. Ralph Morris ENVIRON International Corp Novato, CA T. W. Tesche Alpine Geophysics, LLC Ft. Wright, KY Farmington, NM 16 July 2003. CAMx Overview.

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Technical description of the camx photochemical model l.jpg

Technical Description of theCAMx Photochemical Model

Ralph Morris

ENVIRON International Corp

Novato, CA

T. W. Tesche

Alpine Geophysics, LLC

Ft. Wright, KY

Farmington, NM

16 July 2003

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CAMx Overview

  • Simulates the physical and chemical processes governing the formation and transport of ozone in the troposphere

    • three-dimensional, Eulerian (grid-based) model

    • requires specification of meteorological, emissions, land-use, and other geographic inputs

    • output includes hourly concentrations of ozone and precursor pollutants for each grid cell within a (three-dimensional) modeling domain

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CAMx Overview (continued)

Mathematically simulates the following processes:

  • emission of ozone precursors (anthropogenic and biogenic)

  • advection and diffusion (transport)

  • photochemistry

  • deposition

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CAMx Formulation

Change in Concentration

= Advection by Winds

Turbulent Diffusion

+ Ri + Si + Li


Surface Removal/Deposition

Chemical Reaction

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CAMx Regulatory Applications

  • Regional-Scale Ozone/PM Policy Applications:

    • OTAG

    • Regional NOx SIP Call

    • Tier II Motor Vehicle/Fuel Standards Analysis

    • VISTAS

    • WRAP

    • SARMAP

    • CPRAQS

    • EPA SOx/NOx Regional Transport Rule

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CAMx Regulatory Applications

  • 8-hr Ozone Policy and EAC Applications:

    • Peninsular Florida Ozone Study (PFOS)

    • Missouri/Kansas/EPA (MoKan)

    • Denver EAC

    • San Juan EAC

    • Tulsa EAC

    • Texas EACs

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Comprehensive Air-quality Modelwith eXtensions (CAMx)

  • 3-D Eulerian tropospheric photochemical transport model

    • treats emissions, chemistry, dispersion, removal of gaseous and aerosol air pollution

    • scales range from individual point sources (< 1 km) to regional (>1000 km)

  • Unifies features required of “one atmosphere” “state-of-the-science” models

    • new coding of several industry-accepted algorithms

    • computationally and memory efficient

    • easy to use

    • modular framework permits easy substitution of revised and/or alternate algorithms

    • publicly available (

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CAMx (continued)

Technical Features:

  • Two-Way Grid Nesting

    • horizontal and vertical nesting

    • supports multiple levels

    • variable meshing factors

    • flexi-nesting (any amount of fine grid inputs)

  • Plume-in-Grid (PiG) submodel

  • Multiple, fast and accurate chemical mechanisms

  • Mass conservative and mass consistent transport scheme

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CAMx (continued)

  • Multiple map projections

    • curvi-linear latitude/longitude; Universal Transverse Mercator; Lambert Conformal (MM5); Rotated Polar Stereographic (RAMS)

  • Ozone Source Apportionment (OSAT) and other “Probing Tools”

    • tracks source region/category contributions to receptor ozone concentrations

    • indicates if ozone formed in NOx or VOC-limited conditions

  • Ability to use historical air quality model databases developed for other models

    • OTAG, LMOS, COAST/Houston, Atlanta, Northeast Corridor

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CAMx Technical Components

  • Overview

    • solves continuity equation for each species

    • time splitting operation

      • each process solved individually for each grid, each time step

    • time step size maintains stable solution of transport on each grid

      • multiple transport steps per master grid step required for

        nested grids

      • multiple chemistry steps per transport step required

    • model developed to run on meteorological modeling grid

      • reduces error due to interpolation and averaging

      • multiple map projections available

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CAMx Technical Components (continued)

  • Transport

    • advection and diffusion solvers are mass conservative

    • horizontal and vertical advection linked through the divergent compressible atmospheric continuity equation

      • mass consistency

    • order of east-west and north-south advection alternates each master grid step

    • three options available for horizontal advection solvers:

      • Smolarkiewicz (1983): diffusion-corrective forward-upstream scheme

      • Bott (1989): area-preserving flux-form solver (less diffusive, more accurate)

      • Piece-wise Parabolic Method (PPM)

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CAMx Technical Components (continued)

  • Transport

    • vertical transport solved with a semi-implicit Crank-Nicholson scheme, accounting for:

      • resolved vertical velocity

      • mass exchange across variable vertical layer structure

    • vertical diffusion solved with an implicit scheme

      • dry deposition rates are used as the surface boundary condition

    • horizontal diffusion solved with an explicit scheme in two directions simultaneously

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CAMx Technical Components (continued)

  • Pollutant Removal

    • dry deposition velocities for each species determined using resistance approach (Wesely, 1989)

      • dependent upon: season, land cover, solar flux, near-surface stability, surface wetness, species solubility and diffusivity

      • for aerosols: size spectrum dictates sedimentation velocity

    • wet scavenging based on Maul (1980)

      • exponential decay

      • decay rate dependent upon: rainfall rate, species solubility

      • species removed from entire grid column (all layers)

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CAMx Technical Components (continued)

  • Photochemistry

    • CBM-IV (Gery et al., 1989)

      • 3 variations available

    • SAPRC97 (Carter, 1990) and SAPRC99 (Carter, 1999)

      • chemically up-to-date

      • tested extensively against environmental chamber data

      • uses a different approach for VOC lumping

    • all mechanisms are balanced for nitrogen conservation

    • photolysis rates derived from TUV preprocessor

      • generates lookup table over: zenith angle, altitude, ozone column, albedo, turbidity

      • first two determined for each grid cell internally

      • last three provided by input files

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CAMx Technical Components (continued)

  • photolysis rates affected by clouds

    • UAM-V approach: rates scaled by fractional cloud coverage only

    • RADM approach: rates scaled by optical depth and cloud coverage

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CAMx Technical Components (continued)

  • Chemistry Solver

    • most “expensive” component of photochemical grid simulations

    • CAMx solver increases efficiency and flexibility

    • adaptive hybrid approach:

      • radicals (fastest reacting species) solved using implicitly steady state approximation

      • fast state species solved using second-order Runge-Kutta method

      • slow state species solved explicitly

      • “Adaptive” = number of fast state species changes according to the chemical regime

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CAMx Technical Components (continued)

  • Chemical Mechanism Compiler (CMC):

    • pre-processing program that generates solver FORTRAN source code

    • allows quick, error-free coding of updates or new mechanisms

    • allows for several mechanisms to be available in a single model compilation

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CAMx Technical Components(continued)

  • Aerosol Chemistry (PMCAMx version)

    • gas-phase mechanism (based on CBM-IV mechanism 2)

      • additional biogenic olefin and condensable organic carbon species

      • homogeneous transformation of SO2 to sulfate

      • production of nitric acid

      • production of condensable organic carbon

    • aerosol package calculates the following transformations:

      • aqueous transformation of SO2 to sulfate

      • condensable organic carbon to secondary organic aerosol

      • sodium nitrate formation

      • gaseous nitric acid to aerosol nitrate

      • gaseous ammonia to aerosol ammonium

      • NO3/SO4/NH4 equilibrium as a function of NH4, temperature, humidity

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CAMx Technical Components(continued)

  • Plume-in-Grid (PiG)

    • fine resolution needed for chemistry/dispersion of large NOx plumes

    • tracks stream of plume segments (puffs) in a Lagrangian frame

      • each puff moved by winds in host cell

      • puff growth (dispersion) determined by diffusion coefficients in host cell

    • GREASD PiG: faster, conceptually simpler

      • reduced NOx chemistry set (NO-NO, NOx/ozone equilibrium, HNO3 production)

      • puffs leak mass according to growth rates and grid cell size

      • puffs terminated due to age or sufficiently dilute NOx

    • IRON PiG: slower full photochemistry (not released yet)

      • Incremental Reactions for Organic and NOx (IRON)

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CAMx Technical Components(continued)

  • Ozone Source Apportionment (OSAT)

    • determines source area/category contributions to ozone anywhere in the domain

    • uses tracers to track precursor emissions and ozone production/destruction

    • also tracks contribution of initial and boundary conditions

    • estimates whether ozone is produced under NOx- or VOC-limited conditions

    • removes need to run model repeatedly to understand:

      • chemical regime

      • influences of various sources

    • HOWEVER: cannot quantify ozone response to NOx or VOC controls

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CAMx Technical Components (continued)

  • Ozone Source Apportionment (OSAT)

    • utilizes CAMx routines for dispersion

      • ensures OSAT tracers and CAMx concentrations are consistent with each other

    • 4 basic reaction tracers:

      • NOx emissions (N tracer)

      • VOC emissions (V tracer)

      • ozone production attributed to NOx (O3N)

      • ozone production attributed to VOC (O3V)

    • N and V decay based on weighted NOx and VOC decay rates

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CAMx Technical Components(continued)

  • Ozone Source Apportionment (OSAT)

    • O3N and O3V accumulate a portion of total ozone production/destruction in each cell

      • VOC vs. NOx limited chemistry differentiated by (Sillman, 1995): P(H2O2)/P(HNO3) = 0.35

    • chemical allocation methodologies:

      • OSAT: standard approach

      • APCA: attributes ozone production to anthropogenic (controllable) sources only

      • GOAT: ozone is tracked based on where it formed, not where precursors were emitted

      • OPPAT: ozone formation is attributed to both VOC and NOX that participated

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CAMx Input Requirements

  • Meteorology (Fortran binary)

    • 3-D time-varying fields to define the state of the lower troposphere

    • affects transport, chemistry, surface removal, wet scavenging

  • Air Quality (UAM-IV Fortran binary)

    • initial and boundary conditions

  • Emissions (UAM-IV Fortran binary)

    • 2-D time-varying gridded emission fields and individual

      elevated point sources

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CAMx Input Requirements (concluded)

  • Other

    • user control file (ASCII)

    • chemistry parameters file (ASCII)

    • time- and space-varying vertical grid structure (Fortran binary)

    • landuse / land cover (Fortran binary)

    • albedo / haze / ozone column (ASCII)

    • photolysis rates lookup table (ASCII)

    • OSAT source / receptor definition files (ASCII)

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CAMx Model Output

  • 3-D time-varying average concentration files (UAM-IV Fortran binary)

    • user-selected species

      • ppm (gasses) or g/m3 (aerosols)

    • optionally include surface layer or all layers

    • master grid file and fine grid file

  • 3-D instantaneous concentration files (UAM-IV Fortran binary)

    • all species (mol/m3)

    • all layers

    • output last two hours of simulation for model restart

    • master grid file and fine grid file

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CAMx Model Output (continued)

  • PiG restart files (Fortran binary)

    • all relevant PiG information for model restart

  • Diagnostic files (ASCII)

    • repeat run control parameters and I/O file names

    • diagnostic messages and warning/error messages

    • CPU timing

    • mass budgets

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CAMx Model Output (continued)

  • OSAT output files

    • master and fine grid instantaneous tracer files (Fortran binary)

    • master and fine grid layer 1 average tracer files (Fortran binary)

    • receptor file of tracer concentrations at discrete receptors (ASCII)

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Computer Requirements

  • Dependent upon size of CAMx application

    • standard model vs. large OSAT configuration

    • number of nested grids needed

    • plan accounting for episode length and throughput for desired number of simulations

    • Computer platform used

      • Type (Linux, DEC, SUN, etc.)

      • Clock Speed (MHz)

      • Compiler (optimization)

      • Open MPI Multiprocessors

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Computer Requirements(continued)

  • Minimal Recommended hardware – Unix/Linux workstation

    • 256 Mb memory

      • standard OTAG simulation requires ~75 Mb

      • EPA SIP call OSAT configuration requires ~200 Mb

    • 300 MHZ CPU speed

      • standard OTAG simulation requires ~2 hours/simulation day

      • EPA SIP call OSAT configuration requires ~8 hours/simulation day

    • 10 Gb available disk space

      • standard OTAG simulation requires ~1.5 Gb inputs, 1 Gb output

      • EPA SIP call OSAT configuration requires ~2 Gb inputs, 8 Gb output

    • Graphics monitor and associated drivers

  • ODEQ Linux Computer System

    • Duel 2.2+ GHz Processors

    • 2 Gb of RAM

    • Four 80 Gb drives in RAID system (240 Gb disk storage)

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Computer Requirements(concluded)

  • Minimal Software Requirements

    • Fortran 77

    • PAVE (or some other graphics viewer)

    • ancillary software packages as necessary for pre-processing models

      • SAS (if using EMS-95)

      • Fortran 90 (MM5)

  • ODEQ Linux Computer System

    • PG Fortran compiler (F77/F90)

    • PAVE

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Application of CAMx

  • CAMx Setup

  • Base Case Development

  • Performance Evaluation

  • Future Base Case

  • Emission Control Scenario Evaluation

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CAMx Domain Definition

  • Coverage

    • geographical/political issues

    • influence of boundary conditions

    • needs for fine resolution in key areas

    • depth

    • resource constraints

  • Resolution

    • master grid:

      • met model, coordinate projection, layer structures

    • nested grids:

      • where, how many, vertical nesting

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CAMx Domain Definition(continued)

  • Configuration of nested grids

    • horizontal

    • Vertical (not recommended)

  • Typical application

    • 36/12/4-km grid system

    • Number of vertical layers TBD

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CAMx Domain Definition (continued)

  • Rules for grid nesting:

    1) “meshing factor” must be an integer

    2) cell size of the finest grid must be a common denominator for all parent grids above it

    3) the restriction in (2) does not apply to parallel fine grids of the same generation

    4) fine grids cannot overlap, but can share a common boundary or edge;

    5) fine grids cannot extend into a non-modeled area of the coarse grid;

    6) four “generations” of nests allowed; 10 total grids allowed

    7) the depth of all grids must exactly match

    8) nesting vertical layers is allowed, but they must be a subset of the parent grid layers (vertical nesting not recommended)

    9) there must be a matching fine grid layer interface at each parent grid layer interface

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CAMx Domain Definition (concluded)

  • OSAT source/receptor areas

    • Source Area Map

  • Landuse

    • USGS database, and/or

    • developed from local data used for emission surrogates

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CAMx Air Quality Inputs

  • Initial Conditions (ICs)

    • model spinup

    • role of ambient measurements

  • Boundary Conditions (BCs)

    • Lateral boundaries (time, space varying)

    • Concentrations aloft (time, space invariant)

    • Based on clean air background, observations+clean air, and/or continental model simulations

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CAMx Chemistry Definition

  • “Chemical” Processes in CAMx

    • gas phase chemistry

      • mechanism, rate constants, photolysis rates

    • aerosol chemistry

      • mechanism

    • dry deposition

      • Henry’s law constant (KH), diffusivity, reactivity

    • wet deposition

      • Henry’s law constant

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CAMx Chemistry Definition (continued)

  • Chemistry Parameters File: Contents

    • mechanism (choose from 1 through 5 or inert)

    • photolysis reaction parameters

    • gas phase species list

      • lower bound values

      • deposition parameters (KH, diffusivity, reactivity)

    • aerosol species list (mechanism 4 only)

    • gas phase reaction rate constants

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CAMx Chemistry Definition (continued)

  • Photolysis Rates Input

    • two options:

      • directly input using a look up table, or

      • set by ratio to another reaction

      • must directly input at least one reaction

    • input files:

      • chemistry parameters (specify direct/ratio options)

      • photolysis rate file

      • albedo/haze/ozone column (ALHZOZ) file

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CAMx Chemistry Definition(continued)

  • Photolysis Rate Input Files

    • look-up tables for all directly specified photolysis reactions

    • function of zenith angle, height, albedo, haze, ozone column

    • prepared using TUV light model from NCAR

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CAMx Chemistry Definition (continued)

  • Albedo/haze/ozone column (ALHZOZ) file

    • categorize albedo, haze and ozone column into “bins”

    • bin ranges must match photolysis rate file

    • ALHZOZ contains gridded fields of bin indexes

    • ozone column from NASA satellite data (TOMS)

    • albedo based on USGS land use data

    • haze generally set to a constant default value

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CAMx Meteorological Inputs

  • Objective analysis

    • interpolate measurements to grid points

    • independent fields of winds, temperature, humidity

    • old, rarely used – not recommended

  • Diagnostic models

    • interpolate measurements to grid points

    • adjust gridded fields for effect of terrain, stability, divergence minimization

    • independent fields of winds, temperature, humidity

      • some models provide mixing depth estimates and micromet variables


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CAMx Meteorological Inputs

  • Prognostic models

    • solve predictive equations of motion, thermodynamics, and continuity

    • hydrostatic models => simpler equations, but omit physics at small scales

    • incorporate measurement data to “nudge” model simulation

    • linked, consistent fields of winds, temperature, humidity, and turbulent energy


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CAMx “Probing Tools”

  • “Probing Tools”

    • A module within the photochemical grid model that extracts information on source-receptor relationships, chemical and physical processes, mass flux, etc.

    • Used to diagnose model to understand why it is getting the answer it gets and improve modeling performance

    • Used to better define optimal emission control strategies

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Three “Probing Tools” in CAMx

  • Decoupled Direct Method (DDM) – first-order sensitivity coefficients of ozone by emission source groups (also in URM and CMAQ models)

  • Ozone Source Apportionment Technology (OSAT) – ozone source apportionment by source groups (source category + geographic region)

  • Process Analysis – output information on chemical processes and mass flux (also in CMAQ)

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CAMx Probing Tools

  • Decoupled Direct Method (DDM)

    • Sensitivity coefficients provides information on the relationship of CAMx-estimated ozone (or other species) and sources of precursors (emissions, boundary conditions, and initial concentrations)

    • Information is useful for

      • Control Strategy Development

      • Model performance evaluation

      • Diagnostic analysis

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CAMx Probing Tools (Concluded)

Process Analysis

  • Integrated Process Rates (IPR)

  • Integrated Reaction Rates (IRR)

  • Chemical Process Analysis (CPA)

    Ozone Source Apportionment Technology (OSAT)

  • Ozone contribution by source category and source type

  • VOC vs. NOx limited estimates

  • Anthropogenic Precursor Culpability Assessment (APCA)

    • More control strategy relevant information\

    • Only allocates ozone to biogenic emissions when biogenic VOC interacts with biogenic NOx

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CAMx DDM - Ozone Sensitivity to + 20% Area Source VOC Emissions Changes (DDM) (Brute Force)

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CAMx DDM - Ozone Sensitivity to + 20% Area Source NOx Emissions Changes (DDM) (Brute Force)

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Process Analysis of Odd Oxygen (O3 + NO2) Budget

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Process Analysis:Ozone Time Series Analysis

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Process Analysis: Response of Ozone Production to Emissions

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OSAT Ozone Source Apportionment:Total Ozone and Utility Contribution

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OSAT Ozone Source Apportionment: IndustrialSources and Area+Non-Road Contributions

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OSAT Ozone Source Apportionment: On-RoadMobile and Biogenic Sources Contributions

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OSAT Source Apportionment by Receptor 2007 Base Case

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OSAT Source Apportionment by Receptor 2007 NOx SIP Call